The conditions of use of joints for connecting oil well pipes used in the development and production of petroleum and natural gas are becoming more severe as oil wells and gas wells become deeper, and a higher level of performance is required thereof with respect to strength and airtightness (also referred to below as sealing properties).
In such wells, with threaded tubular joints such as buttress joints or round thread joints which have conventionally been used in which joining is performed solely by threaded engagement of API standard threads, it is not possible to provide either strength or sealing properties adequately.
Therefore, special tubular joints have been proposed which have a metal-to-metal seal portion and which have adequate joint strength and excellent airtightness, and these have come to be actually used.
For example, U.S. Pat. No. 4,732,416 proposes a threaded joint for pipes which can avoid stress concentrations by forming a metal-to-metal seal portion with a shape in which a curved surface contacts a curved surface.
U.S. Pat. No. 4,623,173 proposes a threaded joint for pipes in which a sealing surface on the male thread side of a metal-to-metal seal portion is specified as a convex surface having a radius of curvature of at least 100 mm, and a sealing surface on the female thread side is specified as a sloping surface (a conical surface) having a taper angle of 1.0-4.7°, and the pressure P applied to the metal-to-metal contact portion of the sealing surface (referred to below simply as the “contact portion”) is specified as (the internal pressure applied to the threaded joint)<P<(the yield strength of the material of the threaded joint).
U.S. Pat. Nos. 5,137,310 and 5,423,579 propose threaded joints for pipes in which a metal-to-metal seal portion has a shape in which a conical surface contacts a conical surface.
In JP A 61-124792, a threaded joint for pipes is proposed in which the surface roughness (Ry) of one sealing surface of a metal-to-metal seal portion is specified as 5-25 μm Rmax.
JP A 06-10154 proposes a threaded joint for pipes having a metal-to-metal seal portion which has been subjected to surface treatment to form a surface treatment coating with a thickness satisfying a prescribed relationship with the surface roughness (Ry) (within the range of 5-20 μm) of a substrate.
JP A 07-217777 proposes a threaded joint which aims at improving resistance to seizing (including galling) by making the thickness of a surface treatment coating formed on a metal-to-metal seal portion greater than the surface roughness Rmax (Ry) of a substrate.
JP A 08-145248 proposes a threaded joint of high Cr steel containing at least 10 mass % of Cr in which the surface hardness of one sealing surface of a metal-to-metal seal portion is made at least 10 Hv larger than that of the other sealing surface, and the average roughness of the surface having the higher hardness is made to be in the range of 0.25-1.00 μm.
However, even with special threaded joints having a metal-to-metal seal portion as described above, if the surface roughness of the sealing surface is large, leaks can easily occur. In particular, when the fluid is a gas as with a gas well, it becomes easy for leaks to occur. Therefore, the pressure which it is necessary to apply to the contact portions of the sealing surfaces in order to prevent leaks when the surface roughness is large becomes high. Depending on the state of contact of the sealing surfaces or the conditions of the applied load, it is not possible to apply a sufficient pressure to the sealing surfaces, and the joints may not exhibit desired sealing properties.
With tubular joints having a metal-to-metal seal portion, it should be possible to obtain good sealing properties by reducing the surface roughness of the sealing surfaces and increasing the pressure applied to the contact portions. However, reducing the surface roughness requires a high working accuracy, working time becomes long, and losses from the standpoint of working efficiency and costs become large. On the other hand, when the contact force applied to the seal portion is increased, the pressure becomes high if the width of the contact portions is narrow, and it becomes easy for seizing to take place, leading to damage of the joint.
However, none of the above-described threaded joints having a metal-to-metal seal portion takes into consideration the relationship of the surface roughness and the applied pressure to sealing properties.
For example, the tubular joints for pipes proposed in U.S. Pat. Nos. 4,732,416; 4,623,173; 5,137,310 and 5,423,579 contain no mention of the surface roughness of a sealing surface or the applied pressure.
On the other hand, with each of the threaded joints for pipes described in JP A 61-124792, JP A 06-10154, JP A 07-217777, and JP A 08-145248, the surface roughness is prescribed in the relationship with the surface treatment, and there is no mention of the relationship between the surface roughness and the applied pressure and the width of the contact portion, or of the relationship between these and the sealing properties.
Thus, with special threaded joints having a metal-to-metal seal portion which have been proposed up to now, it was difficult to prevent problems such as seizing or the leakage of fluids with certainty.